The present invention relates to silicon carbide having a high resistivity and a process for its production.
In recent years, along with the progress in the high densification technique and the microfabrication technique of semiconductor integrated circuits, importance of a plasma treatment apparatus such as a plasma etching apparatus or a plasma CVD apparatus has been increasing which is capable of forming a fine circuit pattern in high precision on a semiconductor wafer. Parts to be used for a plasma treatment apparatus include, for example, an electrostatic chuck, a heater, a protective ring, a sleeve and a chamber, and these parts are required to have desired resistivities and have of high purity, high corrosion resistance and uniformity. Among them, the protective ring is required to have a high resistivity for the purpose of carrying out etching uniformly within a wafer, and the sleeve and the chamber are required to have high resistivities for the purpose of minimizing the wearing rate.
Heretofore, parts made of alumina or silica have been used as parts for a plasma etching apparatus, which are required to have high resistivities. However, parts made of alumina have had a problem that high purity products are hardly obtainable, and semiconductor wafers to be treated are likely to be contaminated. Further, parts made of silica have had a problem that wearing by a plasma gas is substantial, such being disadvantageous from the viewpoint of costs. Therefore, in recent years, parts made of silicon carbide have been proposed as parts to be substituted therefor.
On the other hand, as a method for controlling the resistivity of silicon carbide ceramics, a method is known which employs e.g. beryllium, beryllium carbide, beryllium oxide or boron nitride, as a sintering aid (xe2x80x9cSilicon Carbide Ceramicsxe2x80x9d, Uchida Roukakuho, p.327). However, this method has had a problem that since another component is incorporated as a sintering aid, it is impossible to obtain highly pure silicon carbide ceramics, and when used as parts for the semiconductor production apparatus, such parts tend to contaminate semiconductor wafers.
Further, methods for controlling resistivities of silicon carbide ceramics are proposed, for example, in JP-A-52-110499, JP-A-11-79840 and JP-A-11-121311. However, the silicon carbide ceramics obtained by such methods have had problems such that the resistivities are not sufficiently high, the purities are low or the productivity is not good.
JP-A-9-255428 discloses a method for controlling the resistivity of silicon carbide ceramics, which comprises mixing xcex1-type silicon carbide powder, xcex2-type silicon carbide powder and super fine powder of silicon carbide, followed by sintering. However, this method has had a problem such that it is impossible to obtain a product having a large size or a complicated shape, the controllable range of the resistivity is relatively low at a level of not higher than 102 xcexa9xc2x7cm, and it is difficult to obtain silicon carbide ceramics having high resistivities.
Further, JP-A-11-71177 discloses silicon carbide ceramics comprising silicon carbide and silica as the main components and having the resistivity controlled to be from 500 to 50,000 xcexa9xc2x7cm. However, such silicon carbide ceramics have a problem such that silica portions in the sintered body are likely to be selectively eroded by an acid or by a plasma gas, or the obtained sintered body has high porosity and low corrosion resistance.
Still further, JP-A-6-239609 discloses xcex2-type silicon carbide having a resistivity of at least 104 xcexa9xc2x7cm, which is obtainable by a CVD method. However, this xcex2-type silicon carbide has had a problem that the resistivity fluctuates substantially, and it is difficult to obtain a product having a uniform resistivity.
It is an object of the present invention to provide silicon carbide which has a high, uniform resistivity, high purity and high corrosion resistance.
Namely, the present invention provides silicon carbide having a resistivity of from 103 to 106 xcexa9xc2x7cm and a powder X-ray diffraction peak intensity ratio of at least 0.005 as represented by Id1/Id2 where Id1 is the peak intensity in the vicinity of 2xcex8 being 34xc2x0 and Id2 is the peak intensity in the vicinity of 2xcex8 being 36xc2x0.
Further, the present invention provides a process for producing such silicon carbide, which comprises forming xcex2-type silicon carbide on a substrate by a CVD method, then removing the substrate, and heat-treating the obtained xcex2-type silicon carbide at a temperature of from 1,500 to 2,300xc2x0 C.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The silicon carbide of the present invention is formed by a CVD method and has a gas-impermeable dense crystal structure, and it hence exhibits high corrosion resistance against a gas such as CF4 or CHF3 to be used in an etching step.
Further, the silicon carbide of the present invention is characterized in that the powder X-ray diffraction peak intensity ratio represented by Id1/Id2 where is Id1 the peak intensity in the vicinity of 2xcex8 being 34xc2x0 and Id2 is the peak intensity in the vicinity of 2xcex8 being 36xc2x0, is at least 0.005.
Here, the peak in the vicinity of 2xcex8 being 34xc2x0 means the peak at 2xcex8 being 33.2xc2x0xe2x89xa62xcex8xe2x89xa634.8xc2x0, and the peak in the vicinity of 2xcex8 being 36xc2x0 means the peak at 2xcex8 being 35.0xe2x89xa62xcex8xe2x89xa637.0xc2x0.
If the ratio of Id1/Id2 is smaller than 0.005, it is not possible to constantly obtain silicon carbide having a resistivity of at least 103 xcexa9xc2x7cm. Further, if it exceeds 0.5, the effect for increasing the resistivity tends to be small. Accordingly, the ratio of Id1/Id2 is preferably from 0.005 to 0.5, more preferably from 0.007 to 0.2.
Id1 is the intensity of a peak detected as the sum of xcex1-type silicon carbides of 2H structure, 4H structure, 6H structure and 15R structure, and Id2 is the intensity of a peak detected as the sum of xcex1-type silicon carbides and xcex2-type silicon carbides of 2H structure, 4H structure, 6H structure and 15R structure.
It is considered that the larger the ratio of Id1/Id2, i.e. the larger the proportion of xcex1-type silicon carbides, the larger the resistivity.
In the present invention, Id1 and Id2 are values measured under the following conditions by means of a powder X-ray diffraction apparatus. Using CuKxcex1-ray as the X-ray source, the accelerating voltage of the X-ray tube is set to be 40 kV, and the accelerating current is set to be 20 mA. The divergence slit (DS) is set to be 1xc2x0, the receiving slit (RS) is set to be 0.15 mm, and the scattering slit (SS) is set to be 1xc2x0. The sample to be measured is one pulverized to have a particle size of at most 20 xcexcm, so that it is free from orientation.
Further, Id1, and Id2 are peak intensities obtained by smoothing treatment (an adaptive smoothing method to remove background noises, followed by smoothing by a Savitzky-Golay method), followed by removing the background by a Sonneveld method.
As the powder X-ray diffraction apparatus, GEIGERFLEX RAD-IIA, manufactured by Rigaku Denki K.K. may, for example, be employed.
The silicon carbide of the present invention has a resistivity of from 103 to 106 xcexa9xc2x7cm. Silicon carbide having a ratio of Id1/Id2 of at least 0.005 and containing a certain amount of xcex1-type silicon carbide, constantly has a resistivity within the above range.
Further, the above resistivity can be measured, for example, by a potentiometer method by means of a four-terminal resistor.
The silicon carbide of the present invention can be obtained by forming xcex2-type silicon carbide on a substrate by a CVD method, then removing the substrate, and heat-treating the obtained xcex2-type silicon carbide at a temperature of from 1,500 to 2,300xc2x0 C.
The process for producing the silicon carbide of the present invention is characterized in that the xcex2-type silicon carbide obtained by a CVD method, is heat-treated at a temperature of from 1,500 to 2,300xc2x0 C. By carrying out the heat treatment within the above temperature range, phase transfer of the silicon carbide from xcex2-type to xcex1-type can be controlled, and it is possible to obtain silicon carbide having a desired resistivity and little fluctuation in the resistivity. If the temperature is lower than 1,500xc2x0 C., no adequate energy tends to be given to the xcex2-type silicon carbide, and no substantial phase transfer to xcex1-type tends to take place, whereby it tends to be difficult to obtain silicon carbide having a desired resistivity. On the other hand, if the temperature is higher than 2,300xc2x0 C., abnormal particles are likely to form during the phase transfer to xcex1-type, whereby the strength of the obtainable silicon carbide tends to be weak. The heat treatment is particularly preferably carried out at a temperature of from 1,800 to 2,000xc2x0 C.
In the present invention, the resistivity can be controlled by the heat treatment, probably by the following mechanism. Namely, by the heat treatment of the xcex2-type silicon carbide, a part thereof undergoes modification to xcex1-type, whereby a mixture of xcex1-type silicon carbide particles and xcex2-type silicon carbide particles will be obtained. Here, along the grain boundaries of the xcex1-type silicon carbide particles and the xcex2-type silicon carbide particles, electrical barrier walls will be formed to increase the resistivity. Accordingly, as the grain boundaries of the xcex1-type silicon carbide particles and the xcex2-type silicon carbide particles will increase, the resistivity will also increase.
Further, by the process of the present invention, silicon carbide having little fluctuation in the resistivity is obtainable, probably because xcex1-type silicon carbide can be formed as uniformly dispersed by once forming only xcex2-type silicon carbide, followed by heat treatment.
Further, the heat treatment is preferably carried out under a pressure of from 0.1 to 2.0 atms (absolute pressure, the same applies hereinafter), particularly preferably from 0.2 to 1.5 atms. By carrying out the heat treatment under such a pressure, the resistivity can constantly be controlled. If the pressure is lower than 0.1 atm, the silicon carbide tends to be readily decomposed into silicon and carbon.
The time for the heat treatment is preferably from 1 to 100 hours, particularly preferably from 5 to 10 hours. As the time for the heat treatment is long, the resistivity tends to be large. Further, after the heat treatment, the silicon carbide is preferably cooled at a rate of from 2 to 20xc2x0 C./min, particularly preferably from 5 to 10xc2x0 C./min. As the cooling rate is slow, the resistivity tends to be large. Thus, according to the present invention, the resistivity of the silicon carbide can be controlled by controlling not only the heating temperature but also the time for the heat treatment and the cooling rate after the heat treatment.
The heat treatment is preferably carried out in an inert atmosphere of e.g. argon or helium, or in a non-oxidizing atmosphere such as in vacuum. If the heat treatment is carried out in an oxidizing atmosphere, the silicon carbide surface will be oxidized to form silicon dioxide, and it will be necessary to remove the surface by e.g. cutting or grinding after the heat treatment.
Further, in the process of the present invention, after forming the xcex2-type silicon carbide on the substrate by a CVD method, the substrate is removed to obtain the xcex2-type silicon carbide.
Here, the starting material gas to form silicon carbide by the CVD method, may, for example, be a single gas such as methyltrichlorosilane or dimethyldichlorosilane or a mixed gas of silane, disilane, tetrachlorosilane or trisilane, with methane or ethane. The starting material gas is preferably introduced by using e.g. hydrogen, helium or argon as the carrier gas.
When the starting material gas is introduced as diluted by the carrier gas, it is preferred to introduce it by adjusting the molar ratio of the starting material gas to the carrier gas to be from 1:9 to 4:6.
Further, the temperature at the time of forming silicon carbide by the CVD method, is preferably from 1,000 to 1,400xc2x0 C., more preferably from 1,200 to 1,350xc2x0 C. If the temperature is lower than 1,000xc2x0 C., the rate of formation of silicon carbide tends to be slow, and if it exceeds 1,400xc2x0 C., xcex2-type silicon carbide and xcex1-type silicon carbide tend to be present non-uniformly, whereby the resistivity of the obtainable silicon carbide tends to be non-uniform.
As the substrate for forming silicon carbide thereon, silicon carbide, alumina or high purity carbon may, for example, be used. Among them, it is preferred to employ a substrate made of highly pure carbon having a purity of at least 99.99% from the viewpoint of the purity and efficiency in the removal of the substrate.
The silicon carbide of the present invention is preferably one having a total content of metal impurities of not more than 50 ppb. Here, metal impurities include Fe, Cu, Mg, Al, V, Ni, Na, K, Ca and Cr. If such metal impurities are included, the product tends to be an electrically conductive carrier, and the resistivity will thereby be fluctuated. Further, when the silicon carbide is used as a part for a semiconductor production apparatus, such metal impurities tend to contaminate semiconductor wafers.
The content of the above metal impurities can be measured by glow discharge mass spectrometry (GD-MS method).
The silicon carbide having a low content of such metal impurities, can be produced, for example, by using high purity carbon having a purity of 99.99%, treated for purification by a halogen gas, as the substrate to form silicon carbide.